How to Convert Inches to Millimeters: Formula, Chart, and Fraction-Inch Math

To convert inches to millimeters, multiply the number of inches by 25.4, because one inch equals exactly 25.4 millimeters by international definition: 2 inches is 50.8 mm, and half an inch is 12.7 mm. Convert any length instantly with our free inches to millimeters converter, and use the fraction-inch table below for screws, screens, and rainfall.

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The Formula: Inches Times 25.4

The formula for converting inches to millimeters is millimeters = inches times 25.4, and it works for every inch, from screw diameters to screen sizes to rainfall totals. One inch is 25.4 millimeters, two inches is 50.8 millimeters, three inches is 76.2 millimeters, six inches is 152.4 millimeters, and twelve inches is 304.8 millimeters, and the pattern never changes, because the inch is defined as exactly 25.4 millimeters. The multiplication is simple, and the only real skill is keeping precision where it matters, because rounding 25.4 to 25 loses nearly 2 percent, and that distinction is the subject of the mistakes section. For fractional inches, multiply the fraction directly: a quarter inch is 6.35 millimeters, half an inch is 12.7 millimeters, three quarters of an inch is 19.05 millimeters, and an eighth of an inch is 3.175 millimeters. If you are converting the other way, divide millimeters by 25.4 instead, so 100 millimeters is about 3.94 inches and 254 millimeters is exactly 10 inches. A useful mental shortcut is that one inch is roughly 25 millimeters, which is close enough for rough cuts but not for machining, where a 0.4-millimeter error per inch ruins a fit. For exact figures, use the full 25.4 factor rather than the rounded 25, because on a 12-inch part the rounding gap alone is 4.8 millimeters, nearly a fifth of an inch. For the reverse trip, our millimeters to inches converter runs the same definition backward, and the centimeters to inches converter covers the centimeter side of the same ruler.

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How Many Millimeters in an Inch? The 1959 Definition

How many millimeters in an inch is exactly 25.4, and the exactness is recent: before 1959, the US inch and the British inch differed by a tiny amount, about two parts per million, which was enough to matter in precision engineering. The International Yard and Pound Agreement of 1959 fixed the yard at exactly 0.9144 meters, which made the inch exactly 25.4 millimeters everywhere, and every ruler, screw thread, and screen size since then has used that definition. The same agreement is why a foot is exactly 304.8 millimeters and a yard is exactly 914.4 millimeters, so the whole imperial length ladder converts cleanly once you know the one 25.4 figure. In practice, a source that says inches means the international inch at 25.4 millimeters, whether it is a US hardware store, a British engineering drawing, or a Japanese screen specification, because no competing inch definition survives in commerce. The only trap is not the definition but the rounding: a screen advertised as 27 inches is 685.8 millimeters diagonally, and a bolt described as a quarter inch is 6.35 millimeters in diameter, and both figures are exact, not approximate. For related length math, our guide to converting miles to yards and our guide to converting kilometers to miles use the same multiply-and-check habit.

Inches to Millimeters Conversion Chart

An inches to mm conversion chart covers the values people actually look up, and the first few rows are worth memorizing. A sixteenth of an inch is 1.5875 millimeters, an eighth of an inch is 3.175 millimeters, a quarter inch is 6.35 millimeters, three eighths of an inch is 9.525 millimeters, half an inch is 12.7 millimeters, five eighths of an inch is 15.875 millimeters, three quarters of an inch is 19.05 millimeters, seven eighths of an inch is 22.225 millimeters, one inch is 25.4 millimeters, two inches is 50.8 millimeters, three inches is 76.2 millimeters, six inches is 152.4 millimeters, and twelve inches is 304.8 millimeters. That last row matters because a foot is the unit hardware stores still think in, and knowing that 12 inches is 304.8 millimeters lets you convert any foot-and-inch measurement in one step. For values between chart rows, multiply directly rather than interpolating, because a sixteenth-inch estimating error is a 1.5875-millimeter error, enough to matter in a drilled hole. A household example helps the scale feel real: a standard pencil is about 7 inches, or 177.8 millimeters, a smartphone screen of 6.1 inches measures about 154.9 millimeters diagonally, and a credit card is 3.37 inches wide, or 85.6 millimeters. If you need the reverse chart, divide each millimeters figure by 25.4: 50 millimeters is about 1.97 inches, 100 millimeters is about 3.94 inches, and 300 millimeters is about 11.81 inches.

Fractional Inches to Millimeters: The 1/16 and 1/32 Table

Fractional inches to millimeters conversion is where the 25.4 factor earns its keep, because fractions are how wrenches, drill bits, and lumber are actually sold. A thirty-second of an inch is 0.79375 millimeters, a sixteenth is 1.5875 millimeters, an eighth is 3.175 millimeters, a quarter is 6.35 millimeters, a half is 12.7 millimeters, and a whole inch is 25.4 millimeters, and every other fraction is a multiple of the thirty-second: three thirty-seconds is 2.38125 millimeters, five eighths is 15.875 millimeters, and fifteen sixteenths is 23.8125 millimeters. Drill bits are the classic case: a 1/8-inch bit is 3.175 millimeters, a 3/16-inch bit is 4.7625 millimeters, and a 1/4-inch bit is 6.35 millimeters, so a metric drill set that jumps from 3 to 3.5 to 4 millimeters does not exactly match any imperial bit, and the nearest fit is a compromise, not an identity. Lumber is the other case: a board sold as one inch thick is usually surfaced to 3/4 inch, which is 19.05 millimeters, not 25.4, so converting the nominal size instead of the actual size is a second trap on top of the unit conversion. When a drawing gives a tolerance like plus or minus 1/64 inch, that is plus or minus 0.396875 millimeters, a figure worth knowing before you promise a fit. For area math on the same project, our guide to converting acres to square feet covers the large-scale side of the same tape measure.

Inches to Millimeters for Screens, Screws, and Rainfall

Inches to millimeters math lives or dies in screens, screws, and rainfall, where the numbers decide purchases and forecasts. Television and monitor sizes are diagonal inches, so a 55-inch TV measures 1,397 millimeters diagonally, but its width is about 1,218 millimeters and its height about 685 millimeters, because a 16:9 screen's width is about 87 percent of its diagonal and its height about 49 percent. Screws and bolts are sold by diameter and length in inches, so a number-8 wood screw at 0.164 inches diameter is about 4.17 millimeters, and a 2-inch screw is 50.8 millimeters long, which is why metric screw charts list 4-by-50 as the nearest equivalent, not an exact match. Rainfall is reported in inches, and one inch of rain over one acre is 27,154 gallons, so converting 0.5 inches of forecast rain to 12.7 millimeters tells a gardener the depth but not the volume, which needs the area too. Tire sizes mix the systems openly: a 205/55R16 tire is 205 millimeters wide with a 16-inch rim, which is 406.4 millimeters, so the tire world already converts inches to millimeters on every sidewall. Always convert to millimeters before comparing against a specification printed in millimeters, because catalogs never forgive a unit mix-up.

5 Mistakes That Ruin an Inches to Millimeters Conversion

Five mistakes ruin almost every bad inches to millimeters conversion. First, rounding 25.4 to 25, which loses 0.4 millimeters per inch and 4.8 millimeters over a foot, enough to ruin a machined fit. Second, confusing centimeters with millimeters: one inch is 2.54 centimeters, not 25.4, and a factor-of-ten slip is the most common catastrophic error in this conversion. Third, converting the nominal size instead of the actual size, as with lumber sold as one inch but surfaced to three quarters of an inch. Fourth, measuring screen size as width instead of diagonal, which understates a TV by about 13 percent for a 16:9 panel. Fifth, rounding intermediate fractions: converting 5/16 inch by rounding to 0.3 inches first gives 7.62 millimeters, but the exact figure is 7.9375 millimeters, a gap of 0.3175 millimeters that matters in a drilled hole. The fix for all five is the same habit: use the full 25.4 factor, keep fractions exact until the final step, measure the actual part rather than its label, and convert to millimeters as the last step before comparing against a specification. Do that, and the formula itself, millimeters equals inches times 25.4, will never be the part that goes wrong.